The present invention relates generally to optical waveguides.
The use of fiber lasers and fiber amplifier in the optical information technology is widely expanded. In particular, the erbium-doped fiber amplifiers are known, which have been recommended over last few years in commercial terrestrial systems. These systems have achieved a very high development stage with respect to efficiency in resistance against diverse thermal and weather conditions.
For underwater communications and intersatellite connections, the existing boundary conditions in terrestrial applications lead to radiation damage accumulated over several years of their time of use. They can lead to a slow degradation of the performance up to complete loss of laser or amplifier operation.
Color centers (absorbing centers in visible and near infrared spectral region) in the fibers are responsible for releasing electrons from the atoms of the laser or amplifier materials. These electrons are no longer stationary and can be converted to other atoms in the material and on lattice empty locations in long-term stable centers, which have the spectral broad-band (several hundred nanometers) adsorption. The light power adsorbed in these centers is mainly converted into heat and weakens the use signal which is required for maintaining the laser or amplifier operation.
In the recent time various parameters during production (growth) of the fibers have been researched (pulling speed, temperature, initial materials) as well as the influencing factors of the co-doping required for adjustment of the refraction index profile (for example phosphorus, germanium, aluminum) on the radiation resistance of the fibers. This is disclosed for example in xe2x80x9cRadiation-induced Coloring of erbium-doped optical fibers by G. M. Williams, M. A. Putnam, C. G. Askins, M. E. Gingerich, and E. G. Friebele, SPIE Vol. 1791 Optical Material Reliability and Testing 1992; and xe2x80x9cEffect of Natural Radioactivity on Optical Fibers of Undersea Cablesxe2x80x9d by H. Henschel and E. Baumann, Jour. Lightwave Tech, Vol. 14, No. 5, May 1996.
The use of phosphorus has a disadvantageous effect on the radiation strength of fibers, which can reduce the exclusive utilization of geranium against the radiation damages.
Despite this, there are no today convincing solutions for doping with laser-active ions (rare earths with erbium, neodym, ytterbium) for accumulated radiation doses of 50-200 kRAD, which occur during long-time space applications or undersea cables.
The firm Shott suggested passive glasses with Cer-codoping which are not however doped with laser-active ions. These glasses have a relatively low absorptions induced by radiation.
Accordingly, it is an object of the present invention to provide an optical waveguide which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in an optical waveguide which has a core which is doped with laser-active ions, and which is additionally doped with Cer.
By codoping (admixture) of Cer-ions to the conventionally utilized initial material of a fiber culture, the invention makes possible the elimination of absorption induced by gamma and proton bombardment and thereby connected reduction of the output power.
The invention provides a material combination for less radiation-sensitive fiber laser and amplifier, which makes possible the use of such systems in space and other radiation-loaded environments. With the waveguide of the invention the loss of efficiency induced by the radiation has been efficiently limited to approximately 30% (at 100 kRad Co60 dose).
An especial advantage of the invention is that the ion Cer originates from the same chemical group (rare earths) as the laser-active ions. Therefore a doping ability with Cer is always provided when the fiber material can be doped with laser-active ions of the rare earths group.
The efficiency of the Cer-codoping is an object of a further investigation. The cause for the prevention of the color center formation lies however most probably in an engagement of the electrons which are knocked by the radiation action from the atom before they can form a color center. The electrons can be localized on the Cer, or during so-called charge-transfer transition are again transferred back to the initial atom.
The invention can be used for all laser-active ions in fibers of neodym (Nd) erbium (Er) thulium (Tm), holmium (Ho) ytterbium (Yb), praseodym (Pr), and for all fiber initial materials, such as silicate glass, quartz, fluoride glass.